It is known that molten magnesium and magnesium alloy vigorously react with oxygen in the air to form an oxide and to combust. In order to prevent oxidation of molten magnesium and magnesium alloy, there is used a method of applying a protective flux on molten metal, a method of protecting it with an inert gas such as helium, argon or nitrogen, or a method of covering it with a protective gas.
Hitherto, sulfur dioxide (SO2), sulfur hexafluoride (SF6), etc. have been used as protective gases in magnesium and magnesium alloy production steps. The former has a low price, but its use is limited since it is relatively high in odor and toxicity. The latter has widely been used because of low toxicity and ease to handle. Its global warming potential (GWP) is, however, about 24,000 times that of carbon dioxide (CO2), and it has a very long atmospheric lifetime of 3,200 years. Therefore, its emission is limited in Kyoto Protocol.
Various fluorine compounds have been proposed as protective gases alternative to SF6. For example, difluoromethane (HFC-32), pentafluoroethane (HFC-125), 1,1,1,2-tetrafluoroethane (HFC-134a), difluoroethane (HFC-152a), heptafluoropropane (HFC-227ea), methoxy-nonafluoroethane (HFE-7100), ethoxy-nonafluoroethane (HFE-7200), and dihydrodecafluoropentane (HFC-43-10mee) are cited in Patent Publication 1, Japanese Patent Application Publication 2002-541999, and HFC-134a and dry air are recommended therein as a preferable composition. Furthermore, perfluoroketones, ketone hydrides and their mixtures are cited in Patent Publication 2, US Patent Application Publication 2003/0034094, Patent Publication 3, US Patent Application Publication 2003/0164068, and Patent Publication 4, Japanese Patent Application Publication 2004-276116, and pentafluoroethyl-heptafluoropropyl ketone (C3F7(CO)C2F5) is specifically shown therein as an example. Furthermore, boron trifluoride (BF3), silicon tetrafluoride (SiF4), nitrogen trifluoride (NF3), and sulfuryl fluoride (SO2F2) are cited in Patent Publication 5, U.S. Pat. No. 1,972,317.    Patent Publication 1: Japanese Patent Application Publication 2002-541999    Patent Publication 2: US Patent Application Publication 2003/0034094    Patent Publication 3: US Patent Application Publication 2003/0164068    Patent Publication 4: Japanese Patent Application Publication 2004-276116    Patent Publication 5: U.S. Pat. No. 1,972,317